The present invention is directed to inductive proximity sensors in general, and more particularly, to a method of non intrusive inductive proximity sensing of a permeable or magnetic target through a barrier of conductive material.
Inductive proximity sensors typically comprise a core, which may be “C” or Omega shaped, for example, fabricated from a highly-permeable metal, with two inductive coils on bobbins placed over each leg of the core. The two coils are typically wound around their respective bobbins in opposite directions (one wound clockwise and the other wound counter-clockwise) and electrically connected in series. The series connected coils of the sensor are generally driven by an AC voltage at a desired frequency. The generated coil current, which may be monitored by a current sensing device, is commonly used as an inductive output of the sensor. Generally, the inductive output changes value when a target to be sensed moves from a near to a far position with respect to a sensing face of the sensor, and vice versa. There should be a sufficient change in value of inductive output over the span of operating conditions in order to be able to distinguish between the near and far target positions.
Inductive proximity sensors are typically driven at frequencies between 1 KHz and 5 KHz. When attempting to sense the position of a target of magnetic material through a conductive barrier (i.e. non-intrusive proximity sensing) at these frequencies, significant eddy currents are generated in the conductive material of the barrier. These eddy currents directly oppose the inductive currents generated in the sensor and attenuate the inductive output signal of the sensor. This renders the proximity sensor virtually useless for detecting permeable or magnetic targets through a conductive barrier.
The present invention as will be described in greater detail herein below includes a method of non-intrusive proximity sensing of a permeable or magnetic target through a conductive barrier.